The goal of this clinical trial is to learn about, test, and compare JCXH-221 in healthy volunteers. The main aims to answer are:

• To assess the safety and tolerability of the JCXH-221 vaccine in healthy adult subjects
• To identify an optimal dose for the JCXH-221 vaccine in healthy adult subjects
• To assess the humoral immunogenicity of the JCXH-221 vaccine in healthy adult subjects
• To characterize the cellular immunogenicity of the JCXH-221 vaccine in healthy adult subjects

Participants for Phase I will be randomized to either JCXH-221 or placebo.

This is a phase 1 study looking to enroll a total of 72 patients.

For phase 1, two cohorts will be explored (18-64 age group and 65+ age group) for a total of 72 subjects. The subjects will be enrolled and randomized to either placebo or JCXH-221. A low dose of JCXH-221 will be explored vs placebo for each age group first.

Main Inclusion Criteria

• Sex: Male or female; female subjects may be of childbearing potential, of nonchildbearing potential, or postmenopausal.
• Age: 18 years of age or older, at screening.
• Status: Healthy subjects.
• Subjects must have completed the full doses for primary vaccination with an approved SARS-CoV-2 vaccine and may have received booster dose(s), with the last vaccination (can be 2nd dose of primary vaccination or booster dose) having occurred at least 4 months prior to enrollment.

Main Exclusion Criteria

• Current or prior symptomatic or asymptomatic SARS-CoV-2 infection confirmed by an approved or authorized rapid antigen test on Day 1 or within 4 months prior to Day 1.
• Subjects with significant exposure (as defined by current CDC guidance) to someone with laboratory confirmed SARS-CoV-2 infection or COVID-19 with the past 14 days prior to the Screening visit.
• Subjects with fever or signs of acute infection at the time of enrollment and vaccination.
• Subjects who are taking medications that may prevent or treat COVID-19.
• Subjects who received convalescent serum or prior therapeutic antibodies against SARS-CoV-2 within 4 months before Day 1.
• Subjects with history of myocarditis or pericarditis, or with AEs after mRNA vaccination that are in nature and severity beyond the common AEs expected and necessitating medical intervention.
• Subjects with active or suspected immunosuppression, immunodeficiency, or autoimmune disease.

Double blinded study